If I cook chicken breasts in instant pot for a "longer time" would they come out super tender? I'm thinking 12 minutes or so

Time in the pressure cooker will depend on the thickness of the chicken breasts. I'd say 12-15 min on high pressure should give you meat that can be shredded easily (I've cooked a few times following some variation of this recipe, which yields similar results).

Cooking any meat on high heat (or high pressure) for an extended period will both:

• denature more the proteins / collagen making the meat more tender;
• "squeeze" more water out of the meat, making it drier / chalkier in texture.

At 12-15 min range the chicken meat will dry out some, but let it rest in the cooking liquid for a few minutes before cutting and it should be able to re-absorb some of it (or not release more).